Ink jet printing systems are one example of digitally controlled fluid emission devices. Ink jet printing systems are typically categorized as either drop-on-demand printing systems or continuous printing systems.
Drop-on-demand printing systems incorporating a heater in some aspect of the drop forming mechanism are known. Often referred to as “bubble jet drop ejectors”, these mechanisms include a resistive heating element(s) that, when actuated (for example, by applying an electric current to the resistive heating element(s)), vaporize a portion of a fluid contained in a fluid chamber creating a vapor bubble. As the vapor bubble expands, liquid in the liquid chamber is expelled through a nozzle orifice. When the mechanism is de-actuated (for example, by removing the electric current to the resistive heating element(s)), the vapor bubble collapses allowing the liquid chamber to refill with liquid.
In order to achieve sufficiently high printing resolution and printing throughput, typically there are well over 100 individually addressable drop ejectors per printhead chip. In order to enable the addressing and driving of each of a larger number of drop ejectors, it is necessary to integrate driving and logic electronics on the same chip as the bubble jet drop ejectors, rather than needing to make interconnection of one lead per drop ejector to off-chip electronics
There are various families of bubble jet drop ejector designs which may be distinguished from one another according to the relative primary direction of bubble growth and the direction of drop ejection.
In the first family of bubble jet drop ejector designs, the heating element is located within the fluid chamber directly below the nozzle orifice on a substantially planar surface which is substantially parallel to the plane of the nozzle orifice. When the heating element is pulsed, a bubble is nucleated in the fluid above the heating element. The primary direction of bubble growth is upward relative to the heating element. Downward growth of the bubble is not permitted, because of the planar surface on which the heating element resides. Since the nozzle opening is directly above the heating element, the direction of drop ejection substantially coincides with the primary direction of bubble growth.
In the second family of bubble jet drop ejector designs, the heating element is located within the fluid chamber on a substantially planar surface which is substantially perpendicular to the plane of the nozzle orifice. The heating element is laterally offset from the nozzle opening. When the heating element is pulsed, a bubble is nucleated in the fluid above the heating element. The primary direction of bubble growth is upward relative to the heating element. Downward growth of the bubble is not permitted, because of the planar surface on which the heating element resides. Since the nozzle is laterally offset from the heating element and the nozzle opening is substantially perpendicular to the heating element, the direction of drop ejection is substantially perpendicular to the primary direction of bubble growth.
In the third family of bubble jet drop ejector designs, the heating element is located substantially within the same plane as the nozzle opening with the heating element located at the periphery of the nozzle opening. By “located substantially within the same plane as the nozzle opening” it is meant that the heating element and the nozzle opening are both on the same side of the fluid chamber. By “located at the periphery of the nozzle opening” it is meant that the heating element is located laterally offset from the center of the nozzle opening. The heating element or elements may have a variety of possible shapes. The heating element or elements may surround the nozzle opening, or simply be at one or more sides of the nozzle opening. If the plane of the heating element and the nozzle is defined to be above the fluid chamber (see FIGS. 2-5), then when the heating element is pulsed, the primary direction of bubble growth is downward relative to the heating element. Upward growth of the bubble is not permitted, because of the planar surface on which the heating element resides. As the bubble expands, it exerts a pressure on the fluid in the chamber below the heating element. Since the nozzle opening is above the fluid chamber, the direction of drop ejection is upward, which is substantially opposite to the primary direction of bubble growth. This family of bubble jet drop ejectors in which the direction of drop ejection is substantially opposite to the primary direction of bubble growth is called backshooters. It is within the context of the backshooter family of drop ejectors that this invention is described.
In U.S. Pat. No. 4,580,149, Domoto discloses a drop ejector geometry which is related to the backshooter family. In this geometry all heaters are located within one large common ink chamber. Such a configuration will have unacceptably large interactions, i.e. fluidic cross-talk, between nearby drop ejectors. Also, since the bubble growth is not constrained by a chamber, a significant amount of energy will be lost rather than directed toward ejecting a droplet, so that this structure is not very efficient.
In U.S. Pat. No. 4,847,630, Bhaskar et al. disclose a drop ejector configuration which would operate in a backshooting mode. The process disclosed for making the device is to electroform an orifice plate, form an insulating layer on the orifice plate, form heater elements on the insulating layer, form an electrically insulating layer over the heater elements to protect them against the ink and cavitation damage, form chambers by electroforming, and connect the structure to an ink supply. Such a manufacturing process would not be compatible with integration of driving and logic electronics needed to address many drop ejectors.
In U.S. Pat. No. 5,760,804 assigned to Eastman Kodak Company, Heinzl et al. disclose a backshooter printhead having a plurality of ducts formed on the ink supply side of a cover plate of an ink supply vessel, each duct being in fluid communication with a respective nozzle opening on the other side of the cover plate. For some configurations of high resolution printheads having a spacing between drop ejectors corresponding to more than a few hundred nozzles and ducts per inch, providing individual ducts through the substrate for each nozzle may result in the walls between ducts being somewhat narrow for high-yield fabrication.
In U.S. Pat. No. 5,502,471 assigned to Eastman Kodak Company, Obermeier et al. disclose a refinement of the configuration of the backshooter printhead in Pat. No. 5,760,804 (which was filed prior to Pat. No. 5,502,471, but which was issued later). Obermeier et al. disclose flow throttle structures formed as longitudinally extended channels in a material layer between a chip and the ink supply. On the chip are disposed a plurality of ink channels, ejection openings, and the respective heating elements. It is specified that the material layer (in which the flow throttle structures are formed) covers the ink channels furnished in the chip. The function of the flow throttle is to increase the fluid impedance, and thereby to restrict the amount of ink which is pressed backwards in the direction of the supply channels, in order to improve the energy efficiency of drop ejection and also to reduce the fluidic crosstalk with nearby channels. In some applications, it is advantageous to provide fluid impedance for better energy efficiency and reduced crosstalk by other means than longitudinally extended channels in a material layer which covers the ink channels on the chip.
In U.S. Pat. Nos. 5,841,452 and 6,019,457, Silverbrook discloses a variety of bubble jet drop ejecting structures whose common features include a) the integrally forming of nozzles, ink passageways, and heater means on a substrate; and b) the ink supply inlet being on the opposite side of the substrate from the ink ejecting outlet, with a straight-through passageway connecting the inlet and the outlet. Two of the structures disclosed by Silverbrook would be considered to be backshooter devices (FIGS. 12 and 17 of both cited patents). Furthermore, in Pat. No. 6,019,457, Silverbrook discloses an ink passageway whose cross-section is gradually enlarging over a part of its length, with the larger cross-section being nearer the outlet side. Silverbrook cites the following disadvantage with respect to his FIG. 17 backshooter configuration formed by isotropic plasma etch of a substantially hemispherical chamber, followed by reactive ion etching of a barrel passageway connecting the chamber to the fluid inlet: there are potential difficulties with the nozzles filling with ink by capillary action if the angle of the barrel and the chamber are not closely monitored. Silverbrook's fabrication process for his FIG. 12 backshooter configuration is somewhat difficult to implement, in that it requires printing narrow barrel patterns at the bottom of 300 micron deep channels. It is desirable to have means of making backshooter devices with fluid chambers and connecting passageways having higher yield, tighter dimensional control, and better fluidic performance than the structures proposed in Pat. Nos. 5,841,452 and 6,019,457.
In U.S. Pat. Nos. 6,102,530 and 6,273,553, Kim et al. disclose a backshooter type printhead in which two different bubbles are produced in the fluid by heater elements. The first bubble to be formed is at the entry side of the fluid chamber and acts as a virtual valve to provide a high resistance to fluid exiting the chamber toward the ink entry side of the chamber at the time when the second bubble is formed to provide the drop ejection force. Furthermore, the ink chamber fabrication method described by Kim is an orientation dependent etching step which is subsequent to a previous orientation dependent etch of the ink inlet which intersects the chamber. As is well known in the art, orientation dependent etching of intersecting features having different dimensions will cause rapid enlargement of the two features in such a way that it is difficult to provide tight dimensional control. A concern with the virtual valve type of means for providing fluid impedance is the reproducibility and stability of the fluid impedance within the various drop ejectors of one printhead, both initially and after prolonged use, as well as the reproducibility from one printhead to another. Since the fluid impedance affects drop volume, drop velocity, and refill frequency, the stable and reproducible performance of the device may be compromised.
In U.S. Pat. Nos. 6,478,408 and 6,499,832, S. Lee et al. disclose backshooter type printheads having an ink chamber with substantially hemispherical shape, an ink supply manifold, an ink channel which supplies ink from the manifold to the ink chamber, a nozzle plate with a nozzle at a location corresponding to the central part of the ink chamber, and a heater formed on the nozzle plate around the nozzle. The hemispherical chamber is formed by dry etching through the nozzle with an etch gas which etches the substrate isotropically. In the described embodiments, the ink channel is formed in the surface of the substrate also by isotropically etching through a groove which is narrower than the diameter of the nozzle. The depth of the ink channel is less than the depth of the hemispherical chamber. In some embodiments there is a cusp-like protrusion at the intersection of the hemispherical chamber and the ink channel, the protrusion said to serve as a bubble barrier. In some embodiments, a nozzle guide extends from the edge of the nozzle to the inside of the ink chamber. Because the hemispherical chamber and the ink channel are formed by isotropic etching for a length of time, the resultant geometries will be somewhat dependant on parameters such as gas pressure, substrate temperature, and etch time. Uniformity of chamber and channel geometries, both within a printhead and from printhead to printhead may be difficult to achieve. As a result, it may be difficult to achieve a high yield of devices having the desired drop volume, drop velocity, refill frequency and uniformity.
S. Baek et al. in “T-Jet: A Novel Thermal Inkjet Printhead with Monolithically Fabricated Nozzle Plate on SOI Wafer” (Transducers '03, pages 472-475, June 2003), discloses a backshooting drop ejector configuration made by a trench filling technique in a Silicon on Insulator wafer. Sidewalls of a chamber and fluid restrictor are defined by filling a trench in the top silicon layer, while the bottom of the chamber is defined by the insulator layer. Under-heater layer, heater layer with conductor layer, upper heater layer and metal cover layer are deposited and patterned, and a nozzle plate is formed by electroplating. An ink delivery manifold is formed in the bottom silicon layer. Then the ink chamber and restrictor are formed by isotropic etching through the nozzle.